(C++/ QT) Use a switch statement with strings by turning the strings into an int

Question

At this website I found an interesting way to create a switch statement with strings. However, this seems really long and drawn out. I wanted to know if it's possible to turn a particular string into an integer that can be used in a switch statement.

So psuedo code would be something like this

QString str = "spinbox_1";

switch stoi( str )
case 1543:
    //code here
case 2343:
    //code here
case 3424:
    //code here

Did you try making an int from the string and using it in the switch statement? — juanchopanza, Jul 12 '16 at 18:03
Why would you want to do something like this ? The linked solution seems to be so much better. — Arunmu, Jul 12 '16 at 18:06

score 2 · Answer 1 · edited May 23 '17 at 11:58

2

As @Slava mentioned it is not easily possible. The solution provided by author in mentioned link is probably the most practtical solution. But if you for some reason really need to do it other way and convert string into decimal number, you can use hashing metod. Please refer to below cityhash which is widely used (obviously you can use any other hashing function). https://github.com/google/cityhash

This may be duplicate of: How can I hash a string to an int using c++?

edited May 23 '17 at 11:58

Community

1
1

answered Jul 12 '16 at 18:24

nosbor

2,826
3
39
63

Thank you, I'll give this a shot as well. Hashing may work for my situation. – tisaconundrum Jul 12 '16 at 18:40

score 2 · Answer 2 · answered Jul 12 '16 at 19:13

2

Try to look at this solution: https://github.com/Efrit/str_switch/blob/master/str_switch.h

Unfortunately the description of this solution is avaliable only in Russian (at least I can't find one in English). It is based on computing hash of the string in compile-time. The only limitation it has is it supports strings with 9 character maximum length.

answered Jul 12 '16 at 19:13

Bearded Beaver

646
4
21

this is pretty genius, thank you – tisaconundrum Jul 12 '16 at 19:17

shanek21 · Accepted Answer · 2016-07-12T18:15:00.310

1

If I ever find myself in a similar situation, I use a map to define a specific int from the given string.

For Example:

// The string you want to convert to an int
std::string myString = "stringTwo";

// The mapping that you set for string to int conversions
std::map<std::string, int> stringToInt = \
    {{"stringOne"  , 1},
     {"stringTwo"  , 2},
     {"stringThree", 3}};

// Here, myInt is define as 2
int myInt = stringToInt[myString];

Now you could put myInt into a switch case.

edited Jul 12 '16 at 18:15

answered Jul 12 '16 at 18:12

shanek21

67
6

Obviously QString instead of std::string – nosbor Jul 12 '16 at 18:13
1

This is exactly how it is implemented on link provided by author and what he wants to avoid – Slava Jul 12 '16 at 18:13
1

@Slava Right, it is the same. – nosbor Jul 12 '16 at 18:16
This is the same but in hindsight this feels more readable to me. TY – tisaconundrum Jul 12 '16 at 18:39
So if rather than `string` you use `const char*`s as your `map`'s key you can make it `constexpr` meaning that it will be constructed at compile-time, making it nearly as efficient as a switch statement for `string`s. – Jonathan Mee Jul 12 '16 at 19:41

score 0 · Answer 4 · answered Jul 12 '16 at 18:09

0

No, it is not possible to map a string to an integer uniquely in general - there are simply more strings than integers. You may calculate hash which unlikely to collide for 2 different string and then compare them, but it is still possibility that 2 different strings have the same hash, so you have to compare that strings after you check their hashes. This is how std::unordered_set or std::unordered_map are implemented (aka hash_set and hash_map) so you can use them. But you would not use switch() statement.

answered Jul 12 '16 at 18:09

Slava

43,454
1
47
90

1

The OP's intention to use `swich` with `case`es implies that set of possible strings is known at compile time and finite. For any such set it is possible to construct a function that will map strings from that set to integer uniquely – mvidelgauz Jul 12 '16 at 18:13
No it does not imply, if there is such function then just use `switch` on it's result – Slava Jul 12 '16 at 18:14

(C++/ QT) Use a switch statement with strings by turning the strings into an int

4 Answers4